How to Calculate Your Due Date

When you discover you’re pregnant, the next nine months of your life will be spent centered around one important date: your due date. The due date gives you and your husband an indication of when to expect the baby; sets the stage for pregnancy milestones; and will help you track developmental changes in the baby and the normal pregnancy changes. The more accurate your period record-keeping is, the more accurate your due date will be. Couples trying to get pregnant may also be keeping track of dates of possible conception on a calendar, which can also help narrow down a due date.

Step 1

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Determine the average length of your monthly cycle. If you record your period start date monthly, you should be able to go back 6 months to 1 year to determine the average length, which could be 26 to 32 days or somewhere in between. Write down the date of the first day of your most recent period.

Step 2

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Add 280 days to that date, if calculating the pregnancy due date manually. According to the March of Dimes, pregnancy typically lasts 40 weeks (from the date in Step 1), which is 280 days. For an approximate date, add 9 months plus 1 week. Because some months have fewer days than others, adding 280 days will give a more accurate due date. Or use an online pregnancy due date calendar at Marchofdimes.org or Babycenter.com to determine an approximate due date. (See Resources.)

Step 3

Re-check the date by counting back to determine the possible date of conception. Count backwards 14 days from the date you were supposed to start your period. This is the most likely date of conception. Use one of the online calculators to see if the conception date is the same as the one you calculated. If the numbers are off, try to recall if your last period was a real, full period or if it was only spotting. Bleeding in the first trimester is not uncommon, according to the American Pregnancy Association. (See Resources.)

Step 4

The rule of 10; photo by Caffe/Sxc.hu

Consider that 80 percent of women give birth within 10 days of their projected due date, according to Dr. Lawrence Boveri of St. Joseph Hospital. The 10 days could be 10 days before or after the expected date. Highlight your calendar to reflect those 10 days before the date. According to the Medical College of Wisconsin, 5 percent of women give birth on their estimated due date.

Step 5

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Make an appointment with your an obstetrician or your Ob/Gyn as soon as you suspect you are pregnant. The doctor will also calculate your due date. The doctor may prescribe prenatal vitamins, or you can buy them over the counter at the drugstore. The doctor’s staff will set you up for your next appointment and discuss other important dates and appointments you’ll need during your pregnancy. Use a pregnancy calendar at Whattoexpect.com to learn more about your baby’s weekly development and what to expect during each trimester. (See Resources.)